As long as the chain can loop the two largest sprockets (big front, big rear, not threaded through the RD ) with an inch to spare it's long enough.

The other gauge is to shift to big/big with the RD on and see if there's enough slack to bring the lower pulley forward or up an inch or so, meaning that there's enough slack so the chain isn't pulling the RD forward.

Keep in mind that "B" screw adjustment changes the available slack in the chain by bring the RD down and back, so I consider the second test a better indicator than the first. Also remember that the chain also must be short enough for the RD to take up all the slack in the small/small combination.

The chain length on the Evil is OK. If you look closely, there is roughly one inch of slack in the chain (the RD will allow the chain to expand into nearly a straight line). This is probably the shortest one could go on that bike, though.

Also, and this is important, the chain will never have to be longer than it is shown on that bike since it is a hardtail. If that were a FS bike, the chain would be too short since it does not have enough slack to allow the suspension to fully compress. When setting chain length on a FS, it's important to deflate the rear shock so you can examine the chain with the suspension fully compressed.

I looked at the photo and as Pedalphile agrees, you're fine as is. Given that it's 1x9 bike, I probably would have gone with a longer chain working toward the amount tat the RD can take up.

It isn't a matter on one being better than the other, but I prefer to go with longer chains whenever possible, so I can shorten them if I damage a link. I prefer never to lengthen a chain with a splice, and in your case I leave it as and consider a longer chain when replacement time comes around.